1. Field of the Invention
The present invention relates to a powder marking agent and to a method for marking using the marking agent. More particularly, the present invention relates to a novel marking agent containing an insoluble ingredient which reduces the time required to dry a solvent-fixed marking image.
2. Description of the Prior Art
In steel-making plants, etc., it is occasionally necessary to mark production data and shipment destinations on the produced steel sheets, etc. In the past, it has been widely practiced to spray a heat-resistant white paint through a stencil by hand. However, when the products to be marked are at high temperatures, the working environment is uncomfortable to the personnel, and thus it has been desired to improve the marking practice. Moreover, the stencil, usually made from a tin plate, is soiled by the marking paint and must be washed with a washing liquor containing an organic solvent.
Marking methods have been proposed which rely on a disposable paper stencil wherein the stencil is prepared on the basis of information from an electronic computer and automatic spraying is performed through such a stencil. However, the throwaway-type stencil brings about paper waste having the same area as the marking area, and does not fully meet the demand of the users. Another automatic marking method involves spraying a powdery marking agent in a pattern through a plurality of nozzles controlled by an electronic computer, and an electrostatic screen printing process can also be cited. However, all of these methods have many technical disadvantages.
Generally, in a marking method using a powder marking agent, fixing operation is required to permanently fix the marking powder to the marked material. When the temperature of the material to be marked (hereafter "receptor material") is high or when the heat capacity of the material is low, the image can be fixed by heat-softening the thermoplastic resin component of the powder marking agent. However, the receptor material is frequently a material such as a metal plate, tube or slab which has a large heat capacity. When the temperature of the receptor material is lower than the temperature at which the powder is fixed, it is not practical to heat the receptor material to fix the marking powder. Instead, it is desirable to perform "solvent fixation", well known in the fields of electrophotographic or electrostatic recording using a volatile solvent.
Solvent fixation poses several problems in the field of marking. In many cases, the film thickness of the powder image is difficult to accurately control, and accurate control frequently makes the apparatus too expensive for the purpose. As a result, the time required for drying the solvent varies over a wide range as would be theoretically expected from the fact that it is rate-determined by diffusion and is proportional to the square of the film thickness. When there is a sufficiently long period of time from the end of marking to the next manufacturing step this variation can be tolerated. In practice, however, a receptor material such as a steel sheet is frequently subjected to piling with a magnetic chuck, conveying by a magnetic piler in which the marked surface of the receptor sheet contacts a magnetic roller, ultrasonic probing, etc. Thus, the surface of the receptor sheet frequently makes contact with other conveying devices and mechanisms shortly after marking.
When manufacture brings other materials into contact with the surface of the marking pattern and a part of the fixing solvent remains, the image is still mechanically soft and weak and will be damaged and become illegible. When, for example, the fine magnetized iron powder frequently adheres to the surface of a magnetic roller as a result of the adhesion of the iron powder to the image, the image which originally optically contrasts with the background surface of the receptor material will blacken and become illegible.
Since the volatilization of a solvent depends also upon its vapor pressure, the use of solvents having the lowest possible boiling points can lead to shortening of the time required for drying the solvents, and proper heating (by a stream of hot air for example) is also effective. However, for complete fixation, very volatile solvents are difficult to use.
A marking method has been developed which comprises transfer of a pattern of a marking agent across a wide air gap between the member to be marked and an image holding member (see Japanese Patent Application (OPI) No. 74415/77 (DT-OS 2,656,881) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application")) and improvements have been made therein in Japanese Patent Application (OPI) Nos. 126230/77 (DT-OS 2,656,881) and 127454/77 (DT-OS 2,656,881), Japanese Patent Application Nos. 28364/77 (DT-OS 2,811,076; U.S. patent application Ser. No. 886,447, filed Mar. 14, 1978) now U.S. Pat. No. 4,187,774, issued Feb. 12, 1980, 27757/77 (DT-OS 2,811,075; U.S. patent application Ser. No. 886,448, filed Mar. 14, 1978 now U.S. Pat. No. 4,192,232, issued Mar. 11, 1980) and 27756/77 (DT-OS 2,811,075; U.S. patent application Ser. No. 886,448, filed Mar. 14, 1978 now U.S. Pat. No. 4,192,232, issued Mar. 11, 1980).
In Japanese Patent Application No. 37011/77 (Japanese Patent Application (OPI) No. 122431/78), a method of marking is disclosed by the present inventors in which coarse non-softenable particles are used so as to avoid physical adhesion of other materials. It has been found, however, that a magnetic roller frequently holds a large amount of a fine black iron oxide powder, and the fine powder may soil the surface of the marked image due to the shock generated at the time of its contact with the steel material, and therefore this method is not completely effective.